Semiconductor Laser Emits 67-µm Radiation

A quantum-cascade laser developed by researchers in Italy and the UK may herald the shape of things to come. The device, which emits 4.4-THz radiation, demonstrates that semiconductor laser technology is suitable not only for visible and near-infrared applications, but also for much lower frequency (i.e., longer-wavelength) ones.

Sources of terahertz radiation have great potential in medical imaging, environmental monitoring, security screening and telecommunications. But thus far they have been bulky, such as those based on gas stimulated by CO2 lasers; have operated at cryogenic temperatures, such as P-doped germanium lasers; or have offered output powers of only a few microwatts, such as those employing the photomixing of two visible or near-IR lasers, explained Rüdeger Köhler, a researcher on the project from NEST-INFM and Scuola Normale Superiore in Pisa, Italy.

In contrast, a prototype of the new GaAs/AlGaAs laser diode can already produce more than 2 mW of 67-µm radiation at 50 K and at relatively low threshold current densities. The researchers are confident that they soon will be able to fabricate a device that functions at higher temperatures. Moreover, they see no fundamental impediment to such devices operating at room temperature and at wavelengths longer than 100 µm.

The active region of the laser features approximately 1500 semiconductor epilayers, grown by molecular beam epitaxy at Cavendish Laboratory at Cambridge University in the UK, and has a total thickness of approximately 12 µm. Like other quantum-cascade lasers, the device generates radiation based on the electronic transitions that occur in the stacked layers. The confinement thickness of the layers, therefore, rather than the fundamental bandgap of the materials, determines the frequency of the emitted radiation, Köhler said.

Crucially, the researchers had to develop an approach to confine the optical mode in the active region without increasing unwanted absorption. "Conventional waveguides as used in mid-infrared lasers fail at these long wavelengths," Köhler said. "Our idea was to guide the mode along a single, very thin, metallike layer just below the stack of active modules." They thus included in the structure an 800-nm-thick, highly N-doped layer of GaAs that keeps part of the mode in the undoped GaAs substrate while the rest is in the active region, minimizing propagation loss.

Readying the laser for the market depends on the target application, Köhler said. If the team can raise the operating temperature by 30 K, the device would be suitable for use in medical or security imaging, for which portability is not paramount and which could tolerate the use of liquid nitrogen Dewars.

"Clearly, the short-term aims are operation at higher temperatures, operation in continuous-wave mode and the extension to wavelengths longer than 100 µm," he said. Beyond that, the researchers plan to adapt the laser for chemical detection and wireless local datacom, which will require engineering a device that operates at room temperature or with a Peltier cooler.